Keratins

The important molecules synthesized by the skin include keratin, melanin, collagen and glycosaminoglycans.

Keratins are high-molecular-weight polypeptide chains produced by keratinocytes (Fig. 1). They are the major constituent of the stratum corneum, hair and nails. The stratum corneum comprises 65% keratin (along with 10% soluble protein, 10% amino acid, 10% lipid and 5% cell membrane).

Fig. 1 Molecular structure of alpha-keratin.

The molecule forms a helical coil which, if stretched, unwinds irreversibly to produce the beta form. The covalent bonds linking the cystine molecules provide extra strength.

From J Invest Dermatol 2001: 116; 964–969, with permission of Blackwell Publishing.

Keratin proteins are of varying molecular weight (between 40 and 67 kDa). Different keratins are found at each level of the epidermis, depending on the stage of differentiation. Epidermal keratin contains less cystine and more glycine than the harder hair keratin.

Melanins

Melanin is produced from tyrosine (Fig. 2) in melanocytes and takes two forms:

eumelanin, which is more common and gives a brown–black colour

phaeomelanin, which is less common and produces a yellow or red colour.

Fig. 2 Biosynthesis of melanin.

Eumelanin is a high-molecular-weight polymer of complex structure formed by oxidative polymerization. The phaeomelanin polymer is synthesized from dopaquinone and cysteine (via cysteinyl dopa).

Most natural melanins are mixtures of eumelanin and phaeomelanin. Melanins act as an energy sink and as free radical scavengers, and absorb the energy of ultraviolet (UV) radiation.

Collagens

Collagens are synthesized by fibroblasts (Fig. 3) and are the major structural proteins of the dermis, forming 70–80% of its dry weight. The main amino acids in collagens are glycine, proline and hydroxyproline. Collagens are broken down, e.g. in wound healing, by collagenases, of which the matrix metalloproteinases are important. There are over 22 types of collagen; at least five are found in skin: